Many providers of electric and telephone service (hereinafter, "utilities") often bury their pipes or cables (hereinafter, "conveyances") underground both for reasons of safety and esthetics. Often such utilities accomplish utility burial by plowing a trench in the earth, laying the conveyance in the trench, and thereafter covering the trench and conveyance with dirt. During the burial process, the utility typically records both the depth and location of the conveyance so that others seeking to excavate in the same general vicinity can locate the conveyance, thereby avoiding damage during such excavation. At present, utilities burying conveyances record their location relative to existing physical landmarks, such as roads, buildings, or bridges for example. From knowledge of the recorded distance of the conveyance from such a landmark, a contractor seeking to excavate in the general vicinity can locate the buried utility conveyance.
Unfortunately, utilizing a physical landmark as a reference point for locating a buried underground utility conveyance does not always facilitate accurate location of the conveyance. Occasionally, physical landmarks undergo a change. For example, a road may undergo widening or repair that may alter its relative distance to the buried utility conveyance. A building may undergo renovation or even demolition, precluding the ability to utilize such a structure as a point of reference for a location measurement.
Given that burial records may not often yield an accurate indication of the location of an underground utility conveyance, most utilities must physically locate their buried conveyances in order to effect a repair as well as to provide warnings to excavating contractors. Typically, a technician from the utility physically locates buried utility conveyances using a radio signal detector to detect a locating signal impressed on a conveyance of interest. As the technician gets closer to the buried utility conveyance, the locating signal received by the radio signal detector gets stronger. While "electronic" conveyance location in this manner does enable relatively accurate location of a buried conveyance, such location measurements are not infallible. If other underground conveyances are present in the same general vicinity as the buried conveyance of interest, the locating signals sometimes interfere with each other, making it difficult for a technician to know when the location signal detected by the radio signal detector is at its maximum.
U.S. Pat. No. 5,576,973, issued on Nov. 19, 1996, in the name of Alan Haddy, discloses the desirability of combining the electronic utility conveyance location technique discussed above with a Global Positioning Satellite (GPS) measurement system to precisely record the position where the technician electronically located the buried utility conveyance. As disclosed in the '973 patent, a technician first locates a buried object, such as a utility conveyance, using a radio signal detector. After locating the buried object, the technician then utilizes a GPS receiver to determine the positional coordinates of the cable located via radio signal detection to enable re-location of the object more precisely, rather than relying on a physical landmark as a reference point as was done previously. While the technique described in the '973 patent ostensibly facilitates more accurate re-location of a buried utility conveyance, the accuracy of the location measurement is dependent on the ability of the technician to precisely locate the buried utility conveyance electronically in the first instance. As discussed, locating a buried utility conveyance electronically via a radio signal detector does not always yield very high accuracy. Thus, if the position of the buried utility conveyance, as established by radio signal detection, is inaccurate, simply determining, via a GPS receiver, the exact position where the electronic location occurred will not necessarily improve the ability to accurately locate the conveyance.
Thus, there is need for a technique that accurately locates a buried utility conveyance.